Mass spectrometers using quadrupole ion optical devices are well known. A particular example of such instruments is the triple quadrupole mass spectrometer, typically used for tandem mass spectrometry. This comprises: a first, mass-selective quadrupole device, Q1, a second quadrupole device acting as a collision cell, for fragmentation of ions, Q2; and a third, mass-resolving quadrupole mass analyser, Q3. Many examples of instruments of this type are known, such as the TSQ 8000 ® or TSQ Quantum®, manufactured by Thermo Fisher Scientific, Inc. A further quadrupole device, Q0, is sometimes provided for use as a preliminary mass filter, ion guide or fragmentation cell. This can permit MSn operation.
Each quadrupole device comprises four parallel rods, arranged as two opposing pairs of electrodes. Generally, the pairs of rod electrodes have applied to them opposite phases of radio-frequency (RF) voltage and optionally DC voltage. Mass-selective quadrupoles generally have RF and DC applied to the electrodes, whereas quadrupoles acting as collision cells or ion guides typically have RF only applied. However, certain quadrupole devices may have only static voltages applied to them, for instance for beam shaping or an array of static lenses. The rods can have a circular, elliptical or hyperbolic cross-section. Alternatively, the rods can have a rectangular cross-section and are referred to as flat rod electrodes, in a configuration referred to as a flatapole or square quadrupole. The flat rod electrodes can have bevelled or straight edges. In all cases, the rods are elongated and the ions travel along the direction of rod elongation. Typically the rods in one quadrupole device are orientated in a plane perpendicular to the ions' direction of travel in the same way as those of another quadrupole device.
There are examples of instruments in which the relative orientations of the rods have been varied though. For example, in the TSQ Quantum® instrument, the relative orientation of the rods in the Q1 and Q3 devices is the same but is rotated by 45 degrees with respect to the curved Q2 collision cell. Although such rotational changes have been considered, these have been based on experimental trial and error. Moreover, no optimal approach has been determined and no rationale for such an optimisation has been identified. Thus, improving the performance of the mass spectrometer by setting the relative orientation of the rods in the quadrupole ion optical devices has not reliably been possible.